What impedance does a QCL show?

The impedance of a QCL depends strongly on the wavelength it is designed for, the temperature and the mode it is operated, therefore only rough indications can be given here (consult the datasheet of a particular laser for exact behaviour).

Pulsed mode devices have an impedance in the region of 5..50 $\Omega$ up to about half the threshold current, then it decreases to the region of 0.5..5 $\Omega$. When operated at too high current, the impedance can rise again (a condition to be avoided in any case).